We are proposing a clinical investigation of the pathogenesis of insulin resistance (IR) in skeletal muscle and adipose tissue (AT), focusing specifically on the contributions of glucose delivery, transport and phosphorylation. The primary methodology will be dynamic PET imaging, using three tracers that respectively portray the kinetics of glucose delivery, bi-directional trans-membrane glucose transport and glucose phosphorylation. The three tracers are: 1) [ O]-H2O for quantifying tissue perfusion, this portrays the kinetics of glucose delivery from plasma to tissue;2) [11C]-3-O-methyl glucose, a tracer constrained to bi- directional trans-membrane glucose transport;and 3) [18F]-fluoro-deoxy glucose, which like [11C]-3-OMG is transported, but adds the subsequent metabolic step, that of glucose phosphorylation. During the prior funding period, this triple-tracer dynamic PET imaging protocol was implemented in lean, healthy individuals to study the effect of insulin upon each step and with regard to the distribution of control amongst delivery, transport and phosphorylation. These studies reveal a potent effect of insulin to evoke re-distribution of control, the fulcrum for which is robust activation of glucose transport. The robust activation of glucose transport kinetics in turn amplifies the importance of glucose delivery in governing rates of insulin-stimulated glucose uptake into muscle and there are additional findings of the importance of glucose phosphorylation in contributing to the higher insulin sensitivity of oxidative muscle, obtained with in vivo imaging of soleus and tibialis anterior muscles in the research participants. We propose 4 specific aims to apply this methodology to investigate the pathogenesis of IR. The 1st aim is to quantitatively assess the kinetics of glucose delivery, transport and phosphorylation in skeletal muscle in type 2 DM and as compared to obese and lean non-diabetic men and women. We will appraise the contribution of each step to the pathogenesis of IR. We postulate more severe IR in oxidative muscle, with a dual impairment of glucose transport and phosphoryaltion. The 2nd aim is a parallel study in type 1 DM. The 3rd aim is to implement the triple-tracer dynamic PET imaging protocol in adipose tissue (AT), examining normal insulin action in non-obese volunteers and testing whether differences in AT insulin action are present in obese insulin sensitive volunteers compared to obese IR participants and the relation of AT IR to that of muscle and liver. The 4th aim is investigate the effects of a weight loss and physical activity intervention (WL+EX) on AT and muscle IR in type 2 DM, using the triple tracer methodology to assess effects on glucose delivery, transport and phosphorylation in muscle and AT and how each metabolic step and both organs contribute to the integrated action of WL+EX to ameliorate IR. Resistance to the action of insulin to stimulate glucose uptake into muscle is a major risk factor to develop diabetes. This proposed research will identify causes for type 2 diabetes and point to new targets for effective treatment.